Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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S P E C I F I C ~ T I 0 N
This invention relates to a recuperator having an
improved core within the primary heat transfer surface.
The invention is in the broad field of heat ex-
changers primarily in industrial heating where conservation
of fuel has made it increasingly important to improve the
overall efficiency of extracting energy from the stack gases
resulting from combustion. The industrial apparatus ~or
this energy extraction is known as a recuperator.
A common form of prior art recuperator is a multi-
walled cylinder with the flue gas passing upwardly through a
central cylinder and combustion air passing through two or
more of the surrounding annular passageways. The cylinder
wall separating the flue gas and combustion air is the
primary heat transfer surface. With a double pass on the
air side of the heat exchanger, the outer air passage,
although counter current to the inner air passage,serves
primarily to utilize the incoming air as insulation to the
primary heat transfer occurring in the inner air passage.
The inner air passage relies on counterflow convective heat
transfer from the primary heat transfer surface to the com-
bus~ion air. Where the flue gas cylinder is unobstructed
throughout its length, it can be described as having no
core. A core describes a device inserted in the flue gas
passage of a recuperator for the purpose of enhancing heat
transfer. With no core, heat transfer is accomplished
primarily by radiation from the hot gases and convection
due to the flow of gas past the outer wall surrounding the
flue gas passage. Radiation from the flue gas is a function
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of the gas composition, the dimensions of the flue column
and the temperature of ~he flue gas. Heat transfer by con-
vection is a function of the velocity of flue gas relative
to the heat transfer surface.
An example of a primarily convective core is U. S.
Patent ~o. 3,859,040, January 7, 1975, invented by Shefsiek
and Cone and assigned to the assignee of the present inven-
tion. An example of a reradiating core is U. S. Patent No.
3,886,976, June 3, 1975, invented by Kardas et al and
assigned to Institute of Gas Technology. Other U. S. prior
art patents revealed in a novelty search are:
1,128,146 Jacobs
1,172,144 Keairnes
1,858,665 GibQon
2,356,530 Pflock
2,462,838 Brierly
2,726,681 Gaddis et al
2,733,908 Graham
2,910,276 Escher
3,518,908 Singh
3,595,299 Weishaupt et al
3,886,976 Kardas
The invention is the fruit of a combination of
mathematically and experimentally or empirically justified
considerations and the structure attained is an improvement
on the Shefsiek et al and Kardas et al patents, previously
referred to. The heat exchanger core of the invention is
designed to optimize heat transfer by convection within the
constraint of low pressure drop. The effect of the core is
to induce a swirling motion to the heat transfer media, which
swirling motion is accomplished in wide channels with minimal
pressure loss. The core of this invention achieves a com-
bination of reradiative and convective heat transfer which
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which outperforms either of these modes used separately and
still maintains a low pressure loss. Regardless of which
effect, convection or reradiation, is dominant in the heat
exchanger using the cora of this invention, the structure
as hereinafter more fully described is a non-obvious im-
provement over the prior art.
Thus, the present invention is defined as a recuperator
comprising a cylinder for flue gases constituting a prlmary heat
transfer surface, a core within the cylinder having a plurality
of radial leaves extending outwardly to the cylinder thereby
subdividing the same into a plurality of separate flue gas
passageways each of which in cross section is bounded by two
straight sides and an arcuate side, the radial leaves after
receiving heat from the flue gases serving to reradiate the
heat to the primary heat transfer surface, and a series of
longitudinally spaced triangular baffles in each of the passage-
ways, each baffle secured to the adjacent leaves, inclined to the
horizontal,the baffles serving to interrupt the passageways and
cause the flue gas to have a gentle swirling motion into and out
of the spaces between the outer edges of the baffles and the
cylinder, thereby enhancing the convective heat transfer between
the flue gas and the primary heat transfer surface.
In the drawings:
Figure 1 is a sectional elevation showing diagram-
mically the core of this invention as a replacement for the
core of U.S. Patent No. 3,859,040.
Figure 2 is a transverse section on line 2-2 of
Figure 1.
Figure 3 is a side view of the core in perspective.
Figure 4 is a transverse section on line 4-4 of
Figure 3.
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Figure 5 is a plan view Gf a baffle plate before
assembly.
DESCRIPTION OF A PREFERRED EMBODIMENT
10 represents the cylindrical wall surrounding the
flue gas passage P and constitutes the primary heat transfer
surface from the hot flue gas to the gaseous medium to be
heated. The improved core C of this invention fits within
the cylindrical wall lO and consists of a plurality of
radially extending leaves 12 welded together to form an
integrated structure. The outer edges 16 of the radiating
leaves are in close proximity to the cylinder wall lO. These
leaves 12, which themselves function as a reradiating core,
subdivide the cylindrical passage P into a plurality of flue
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passages 13 of generally triangular cross section bounded by `
the wall surfaces of the leaves 12 and an outer segment of -~
the surrounding wall 10.
In order to enhance the heat transfer by convection,
each of the triangular passages 13 is intercepted by a series
of angularly directed triangular baffle plates 14. Each
baffle has an outer straight edge 15 extending between the
two outer edges 16 of the adjacent radiating leaves 12. The
other two edges 17 and 18 of the triangular baffle abut the
sides of the radial leaves and extend inwardly to the center
of the structure.
When the core 11 of this invention is installed
within a vertically placed cylindrical wall 10, it can be
secured thereto by tabs 19 which engage upper edge 20 of
cylinder 10, permitting the core to be held in place by
gravity. The segmental spaces 21 between cylinder wall 10
and outer edges 15 of the baffles form a circumferential
series of uninterrupted flue gas passageways extending up-
wardly through the recuperator. Because of the baffles
radially inward of the~e segmental spaces 21, the flue gas
passing upwardly is given a swirling motion as indicated by
arrows 22 in Figures 1 and 4.
The invention is not limited to particular sizes
or numbers of baffles, or angular inclination of the baffles,
but the following specific examples are illustrative.
Example 1
The new core of the invention consists of six
leaves 12 making the angle between them 60. The inside
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d iameter of cylinder 10 is 9 inches and the length is 4 feet.
There are six evenly spaced levels of baffles 14 and with
six baffles to each level, this gives a total of thirty-six
baffle plates. Each leaf 12 is of 28-gauge plate 4~15/16
inches wide by 4 feet long, welded along its length to form
a star-shaped structure with 60 between each plate. Plates
are made of alloy material to withstand temperatures of hot
flue gas. The outer diameter of assembled leaf plates is
held within 8-3/4 inches.
Each baffle 14 is cut from sheet metal to form a
triangle as shown in Figure 5. The base 15 of the triangle
is 5-3/8 inches, while the other sides 17 and 18 are each
3-9/16 inches. The triangles are welded equally spaced along
the length of the plates, the long side being opposite the
angle formed by the junction of the plates. All triangles in
a section are parallel.
When the baffles are installed, the angle of inclin-
ation with the horizontal is 60. When this core was tested
in a 9 inch recuperator and compared with prior art cores,
20 preheat achieved was significantly higher for comparable flue
gas exhaust temperatures. At 1800 F the core of the example
achieved a preheat of approximately 850 F, whereas a standard
reradiating core achieved only 760 F. This 90 F higher
preheat represents an improvement of 10. P/O in fuel savings
with only a slight increase in pressurization of the gas-fired
radiant tube over that obtained with a standard core.
ExamPle 2
Following the same principles as shown in Example 1,
a core was made and tested in a recuperator having a 24 inch
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diameter with apprOximately the same improvement in fuel
savings as set foxth in Example 1 for a 9 inch diameter re-
cuperator.
The new core of this invention has also been made
and tested for 6 inch and 14 inch recuperators.
ExamPle 3
In order to understand the use of the improved
core C of the present invention as so far. described, Figure 1
has shcwn in outline form a recuperator connected to a metal-
lurgical furnace heated by a gaq-fired radiant tube. Figure 1
shows a recuperator R and a gas-fired radiant tube type fur-
nace F similar to that described and illustrated in Patent
No. 3,859,040. Instead of the ceramic core 62 of square
cross section as shown in Figures 3, 4 and 5 of said Patent
No. 3,859,040, the present invention uses the improved core
C described herein.
The invention as herebefore described is shown in
connection with specific examples which have been built and
tested. However, the invention is susceptible of various modi-
fications. For example, instead of dividing the core length
into six evenly spaced levels with a total of thirty-six
baffle plates as set forth in Example 1, the core length may
be divided into five to nine portions. Also, the angular
arrangement of baffles, instead of being 60, may be varied
be~ween 45 and 70 with the horizontal
Operation of the Invention
The new core of this invention causes heat transfer
by both convection and reradiation. In the prior art, heat
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transfer by convection has been accomplished primarily by
increasing flow turbulence, but as a consequence there has
always been a substantial pressure loss. Typically, any
significant improvement in convective heat transfer has
always been associated with hiqh pressure losses. In the
new core, the deflecting baffles are so arranged as to gently
channel the flow of hot media into a swirling pattern, there-
by enhancing convective heat transfer with a low pressure
loss.
Reradiation is a function of reradiating surface
area and a configuration parameter known as shape factor.
The baffles in the new core added to the multiple leaf
reradiator increase both surface area and shape factor. A
reradiating core as described in the Institute of Gas
Technology Patent No. 3,886,976 accepts heat by radiation
and convection from the hot media and reradiates energy to
the heat transfer surface separating the hot and cold media.
However, all the energy radiating from the core does not
strike the heat transfer surface. Some radiation from the
core, and the hot gas, is lost by radiation, directly or
reflected, traveling out of the heat exchanger.
The baffles of the present invention inserted into
the leaves of the reradiator tend to reflect radiation that
would otherwise be lost to surroundings back to the heat
transfer surface. This results in an increased shape factor
and is capable of being shown through accepted mathematical
representation.
The heat exchanger core of the present invention is
comprised of a vertical six-leaf base configuration to which
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are attached ba~fle plates so as to divide the core length
into five to nine portions. These baffles are arranged so
that baffles in adjacent leaf sections have ~taggered inter-
cepts with the leaves. The baffles are oriented at an angle
to the axis of the leaf configuration. This angle is vari-
able from 45 to 70 degrees with the horizontal, or transverse
to the leaf axis.
This configuration is inserted along the longitudi-
nal axis of a circular heat exchanger and interior to the
primary heat transfer surface. The core provides heat trans-
fer enhancement from the hot media, flowing interior to the
circular heat transfer surface and around the core in a cir-
cular motion, to the heat transfer surface and hence the cold
media.
The core allows pressure loss on the hot media side
of a heat exchanger to be kept at a low value. For the use
of combustion products as the hot media, pressurization of
the combustion volume is held to a low value to prevent sig-
nificant effluence of combustion products through openings
normally at a slight positive or negative pressure.
From the above it will be seen that the improved
core of this invention gives improved results both from convec-
tion and reradiation.
